While no other dancing master that I have found includes the waltz-galop by name, the accenting of a waltz step done in duple rather than triple time is discussed by Allen Dodworth in
Dancing and its relations to education and social life(1885, reprinted several times through 1900) and is included in dances such as the turn-of-the-century Pasadena, and the idea that waltz steps can be transposed from triple to duple and vice-versa dates back as far as the sauteuse waltz of the first quarter of the 19th century.

Gilbert uses the waltz-galop frequently in variations for both the galop and the schottische, where it typically replaces the second ("step-hop") part. Cross-checking its use in these dances, it is clear that the rhythm is "1&2." The waltz referenced in the step is the "new" waltz of the late 19th century, with strong backward or forward steps or leaps on the first beat of each measure. This is the waltz that evolved into today's Viennese waltz. But the version given by Gilbert in 1890 and his contemporary, Allen Dodworth, in 1885, is rather livelier than the gliding waltz of modern Dancesport, involving a jeté-glissé-coupé sequence that actually puts the dancers in the air twice per bar. Transposed to galop (duple) time, the movements for the gentleman (lady dances opposite) would be:

1 Leap backward along the line of dance onto left foot& (quarter turn clockwise and) Slide right foot sideways along line of dance2 Close left foot to right, displacing the right foot into fourth position forward ("cut") (making quarter turn clockwise)

1 Leap forward along the line of dance onto right foot
& (quarter turn clockwise and) Slide left foot sideways along line of dance
2 Close right foot to left, displacing the left foot into fourth position backward ("cut") (making quarter turn clockwise)Repeat from beginning

The sequence may be easily reversed by leaping forward rather than backward on the left foot and backward rather than forward on the right foot, using counter-clockwise quarter turns.

While he did not use the actual term waltz-galop, Dodworth did describe an application of the waltz step to polka time which is exactly the same as that given by Gilbert for his waltz-galop. He also gives an interesting version for galop time that moves the waltz step relative to the music so that it crosses measures and uses a "12& or "&12" rhythm:

1 Slide left foot sideways along line of dance2 Close right foot to left, displacing the left into fourth position backward ("cut") (making quarter turn clockwise)& Leap backward along line of dance onto left foot(quarter turn clockwise and)1 Slide right foot sideways along line of dance2 Close left foot to right, displacing the right foot into fourth position forward ("cut") (making quarter turn clockwise)& Leap forward along line of dance onto right foot(quarter turn clockwise and)Repeat from beginning

This sequence can be reversed in the same way as the other.

While this latter transposed-waltz-step-in-galop-time of Dodworth's is an interesting galop variation in its own right, when the waltz-galop appears in dance variations in Gilbert's manual, Gilbert's own version is of course the one that should be used.